Volumetric titration is a routine method in many industrial laboratories for the quantitative determination of the quantity of a substance or an analyte in a fluid sample. Traditionally, a titration reagent is added from a burette to a sample, which latter is stirred, and the change, for example in the pH or the conductivity of the sample, is monitored using appropriate sensors. The unknown concentration or the quantity of the analyte in the sample can be determined with the aid of the volume of titration reagent consumed and the titer of the titration reagent by the end point or equivalence point of the titration and the stoichiometry of the reaction which has occurred. The term “titer” is used to describe the quotient of the actual concentration of a titration or measurement solution and the target concentration of that same solution. Thus, the titer is a factor in the characterization of normal solutions.
Nowadays, instead of manually operated burettes, automated titration devices are used; they are also known as titrators. Titrators comprise at least one automatic dosing element by means of which the titration reagent is added to a sample in predetermined increments or dynamically, at least one appropriate sensor, a control unit and a display unit. Furthermore, generally the titration reagent in a titrator is also fluid and it is necessary to inspect the titer of the titration reagent regularly and/or to prepare fresh titration reagent in order to avoid changes based on aging and/or contamination, because these lead to errors in the measurements.
In addition to volumetric titration, coulometric titration or coulometry is also known for the quantitative determination of the amount or concentration of an oxidizable or reducible compound. A coulometric titration cell comprises two electrochemical half-cells, wherein one electrochemical half-cell functions as the working electrode and the other electrochemical half-cell functions as a counter-electrode disposed in an electrolyte. In coulometry, the titration reagent is produced electrochemically at the working electrode during the titration and the electrical charge which is generated at the working electrode is determined. The reverse electrochemical process occurs at the counter-electrode, wherein the electrolyte within the electrochemical half-cell of the counter-electrode is consumed during the titration because of the reduction or oxidation of the substances contained at the counter-electrode. The two electrochemical half-cells of the coulometric titration cell can be separated from each other by a diaphragm which permits the transport of charge and material between the two electrochemical half-cells as a function of the type and polarity of the working electrode; transport through the diaphragm may be in one or both directions.
Depending on the configuration of the coulometric titration cells, acid or basic titrations may be carried out, for example, and the coulometric titration cell may be configured as a base or acid generator, whereby a base generator produces hydroxide ions (OH−) for acid titration and an acid generator produces hydroxonium ions (H3O+) for base titration.
Published application WO 2009/076144 A1 discloses, for example, a coulometric titration cell with a platinum working electrode, a platinum counter-electrode and a multi-layered ion exchange membrane as the diaphragm. When a suitable electrolyte and a suitable ion exchange membrane are employed, the coulometric titration cell can be used for acid titration or for base titration depending on the polarization of the electrodes. As an example, for acid titration, an aqueous sodium hydroxide solution (NaOH) together with a cation exchange membrane may be employed.
Furthermore, for coulometric titrations, other substances may also be produced in situ, which then react with the analyte in the sample. If the sample contains iodide, then this can be reduced at the working electrode to iodine and then, for example, sulphur dioxide (SO2) in the sample can be determined.
However, known coulometric titration cells suffer from the disadvantage that when carrying out the titration, gases are formed at the working electrode and/or the counter-electrode which have to be removed from the coulometric titration cell; also, they are often produced from precious metals and so are expensive to produce. In particular, the preparation of smaller and more compact coulometric titration cells has not previously been possible.
The objective of this invention is to provide a coulometric titration cell which is small and compact in design and which essentially does not produce any gases during operation.